Recent development of portable electronic devices such as mobile phones, notebook computers, and the like lead to rapid growth of demand for secondary batteries to be employed as power sources thereof. Furthermore, to reduce problems caused by global warming and depletion of fossil fuels, hybrid electric vehicles (HEV) and electric motor-driven electric vehicles (EV) have been developed and secondary batteries are increasingly adopted as power sources thereof. Accordingly, various studies have been conducted to provide secondary batteries capable of satisfying various requirements. Particularly, demand for lithium secondary batteries having high energy density, high discharge voltage and output is increasing.
In order to be applied to electric vehicles and the like, a lithium secondary battery needs to have high energy density and to provide high output in a short period of time while ensuring operation over 10 years or more under severe rapid charge and discharge conditions. Thus, it is necessary for such a lithium secondary battery to ensure superior output and longer lifespan than existing small-sized lithium secondary batteries. Further, since the lithium secondary battery for electric vehicles can undergo rapid exothermic reaction, it is necessary to prevent rapid exothermic reaction from occurring therein to secure safety.
A lithium secondary battery generally has a structure wherein a non-aqueous electrolyte containing a lithium salt is enclosed in an electrode assembly of a positive electrode and an negative electrode each having an active material coated on a current collector, with a porous separator interposed between the positive electrode and the negative electrode. The positive electrode active material is generally composed of lithium cobalt oxides, lithium manganese oxides, lithium nickel oxides, lithium composite oxides, and the like, and the negative electrode active material is generally composed of carbonaceous materials.
Carbonaceous materials are generally classified into graphitizable carbon (soft carbon) that has a graphene structure, non-graphitizable carbon (hard carbon), and graphite which has a complete graphene structure.
In particular, considering a theoretical maximum capacity of 372 mAh/g, an negative electrode composed of a graphite material of the graphene structure predominantly used as an negative electrode active material has a limit in capacity increase and thus is not ideal as a power source for future mobile devices.
On the other hand, although lithium has been suggested as an negative electrode material of the secondary battery, lithium has a problem of low reversibility. When lithium ions of the electrolyte are deposited onto the lithium metal of the negative electrode in a charging process, only a portion of the deposited lithium ions are dissolved into the electrolyte in a discharging process. That is, since only a portion of the lithium ions deposited onto the ions during the charging process can be reused during the discharging process, a relatively large amount of lithium is needed in order to prevent capacity deterioration. Further, the lithium ions are deposited in a dendrite or needle structure on the surface of the lithium material. Here, the dendrite lithium crystals can penetrate the separator to come into contact with the positive electrode, causing internal short circuit. Such short circuit can trigger exothermic breakdown, causing explosion of the battery.